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Versions: 00 01                                                         
Network Working Group                                        J. Peterson
Internet-Draft                                             NeuStar, Inc.
Intended status: Standards Track                             C. Jennings
Expires: September 15, 2011                                Cisco Systems
                                                          March 14, 2011

        The Advertisement/Proposal Model of Session Description


   In common SIP practice, a two-phase "offer/answer" exchange of
   session description documents negotiates preferences, capabilities
   and requested sessions.  However, the structure of the session
   description greatly confuses the disambiguation of these elements and
   thus the clear characterization of sessions.  The current work
   proposes an alternative to the offer/answer model which leverages
   pre-association between user agents to recast those two phases into a
   less ambiguous form: an Advertisement of capabilities and preferences
   which occurs in non-real-time before a session is ever requested,
   which is followed during session establishment by a unidirectional
   and complete Proposal of a session.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 15, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Limitations of the offer/answer model  . . . . . . . . . . . .  5
   4.  Overview of the Advertisement/Proposal Model . . . . . . . . .  6
   5.  Benefits of the Advertisement/Proposal Model . . . . . . . . .  8
   6.  The Advertisement  . . . . . . . . . . . . . . . . . . . . . .  9
   7.  The Proposal . . . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Backward Compatibility . . . . . . . . . . . . . . . . . . . . 10
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   11. Informative References . . . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Terminology

   In this document, the key words "MAY", "MUST, "MUST NOT", "SHOULD",
   and "SHOULD NOT", are to be interpreted as described in [RFC2119].
   This document additionally uses [RFC5226] language to describe IANA
   registrations.  The terms "watcher" and "presentity" and related
   presence terms are defined in RFC2778.

2.  Introduction

   The Session Description Protocol (SDP) [RFC4566] enjoys widespread
   use in the Internet for the description of sessions established by
   protocols such as the Session Initiation Protocol (SIP) [RFC3261].
   Most uses of SDP assume an offer/answer model as described in
   [RFC3264] for interactive session establishment, where one side sends
   an offer and the other side replies with an answer.  However, many
   important matters relating to the management of media streams in SDP
   are inherently ambiguous in the offer/answer model.  In particular,
   distinguishing the expression of capabilities from preferences from
   actual desired media streams has proven extremely difficult for more
   advanced applications of SDP.  Moreover, as the offer and answer
   reflect only the media sinks of their respective creators, neither
   offers a complete picture of the session, and the degree to which the
   answer qualifies the offer admits of similar crippling ambiguities.
   These core difficulties have motivated numerous proposals to reform
   SDP, most of which radically increase its syntactical complexity, and
   thus have found mixed acceptance in the deployment community.

   Perhaps the root of these problems lies not in SDP itself, however,
   but in application of SDP to the offer/answer model.  SDP
   significantly predates the codification of the offer/answer model,
   and while RFC3264 added a great deal of value by formalizing the
   previously tacit principles of using SDP in SIP, nonetheless those
   principles themselves have intrinsic limitations.

   This document therefore proposes a reexamination of the fundamental
   requirements underlying the use of SDP in session-establishment
   protocols such as SIP.  It recognizes, first of all, the desirability
   of conveying in a SIP INVITE message a document that describes the
   proposed session completely, hereafter a Proposal.  In order to
   formulate such a Proposal, however, the originator of a session must
   have access to the capabilities and preferences of the target.  Thus,
   in an alternative model entities willing to receive a Proposal might
   promulgate Advertisements to would-be proposers.  Advertisements
   contain all of the information about the advertiser required by
   proposers to formulate a Proposal, including details such as the IP
   address and port where they hope to receive media.  One way that

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   advertisements might deliver Advertisement to proposers is through

   Several previous efforts have recognized the need to provide
   capability and preference information prior to commencing the session
   establishment process.  Notably, the work on expressing user agent
   capability within presence information [RFC5196] (which in turn
   builds upon the user agent capabilities work in [RFC3840]) provides
   one possible framework in which, with some enhancements, an
   Advertisement might be articulated.  Work has also been done in
   [RFC3407] to express capability information with less ambiguity in
   SDP; by going down that path, an Advertisement might even be rendered
   in SDP.  Rendering a Proposal in SDP would probably return that
   format to something like its original purpose.  This document,
   however, proposes no specific syntactical format for Advertisements
   or Proposals - it merely describes the semantics of these objects as
   a set of requirements that future format proposals could meet.

   While the work in this document is most immediately relevant to SIP,
   and discusses its applicability to SIP throughout, the underlying
   model may have further applicability to other protocol using session
   descriptions in a similar manner.

3.  Limitations of the offer/answer model

   There are several limitations of the offer/answer model that cause
   issues with the operation of SIP.  Many have been the target of prior
   work intended to repair the problem by adding additional syntactical
   structure to SDP.  However, these problems arise from the semantics
   of SDP in the offer/answer context - not only because an offer
   represents an incomplete picture of the session, and at that a
   picture filled with conditional elements, but because an answer does
   not describe a session completely either.

   In the traditional offer/answer model, in order to maximize the
   chances of the negotiation process arriving at an acceptable session,
   an offerer must propose the largest set of session features possible.
   This includes the various combinations of RTP profiles, transport
   addresses, security properties and so forth that would be acceptable
   to the offerer.  Disambiguating the semantics of these features -
   distinguishing, for example, the session features the offerer would
   like to invoke concurrently as opposed to those offered as
   alternatives - has required the additional of significant complexity
   to SDP.  Because SDP's underlying syntax is not designed to carry
   structured or hierarchical data, the resulting documents can be large
   and confusing.  Indeed, the term "offer" no longer seems to fit what
   that sort of document describes - it is a menu rather than a

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   selection.  The resulting negotiation process, in which an answerer
   evaluates the alternatives described in the offer and arrives at a
   response, has become complex enough to call into question the wisdom
   of executing it in real-time, such as during the alerting phase of a
   telephone call.

   Architecturally, an offerer moreover cannot anticipate what entity,
   or entities, their offer will reach, due to the potential in SIP for
   retargeting and/or forking.  The fact that an answer typically
   arrives in a message confirming session establishment (the 200 OK),
   yet that it can originate from a surprising source, and make
   surprising stipulations, is a weakness in offer/answer.  The offer/
   answer model places all of the onus, and the authority, on the
   answerer to complete a session within the very broad constraints that
   can be specified by the offerer.  It is unclear why this power is
   better invested in the answerer than the offerer, but ideally,
   whichever side develops a finalizes proposal for the session should
   pass it to the other for approval, rather than assuming it will be
   acceptable and turning on the session.  That somewhat philosophical
   point is not purely theoretical - it is one of the major drivers for
   classic problems in SIP such as early media, where due to forking
   several answerers may unknowingly operate on the same offer at once.

   The fact that neither an offer nor an answer alone contains a
   complete picture of the session also compels intermediaries
   interested in SDP, for example application-layer gateways assisting
   with NAT traversal, to hear and modify, potentially, both requests
   and responses (e.g. a 200 carrying SDP) during session establishment.
   Unlike requests, responses cannot be rejected at the SIP layer, for
   security reasons or any other reasons, which effectively prevents
   endpoints from cleanly negotiating away from any unwanted
   "suggestions" made by intermediaries.

   Some of this problem space in SDP is addressed by ongoing work of the
   MMUSIC working group on capability negotiation in SDP.  The
   Advertisement/Proposal model differs from the capneg work in so far
   as capneg retains the offer/answer model and provides sufficient
   syntactical equipment in SDP to differentiate the various functions
   of expressing capability, proposing the properties of a session and
   confirming those proposals.  However, despite their differences the
   approaches here and in are not mutually exclusive; in order to
   express Advertisements or Proposals in SDP, something very much like
   the attributes defined in the capneg work is required.

4.  Overview of the Advertisement/Proposal Model

   An Advertisement is a document detailing the parameters of sessions

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   that a user is willing to accept.  A user agent who aspires to
   initiate a session uses an Advertisement as the basis for a Proposal,
   a complete description of a session, by selecting a common set of
   desired capabilities from the Advertisement.  This Proposal is then
   carried in a session initiation message such as an INVITE whose
   recipient will either accept or reject it in its entirety.  A
   recipient who rejects a proposal may search for an Advertisement from
   the proposer, and in turn make their own Proposal based on it; this
   counter-Proposal has no necessary relationship with the rejected
   Proposal.  Unlike the offer/answer model, in the Advertisement/
   Proposal model a Proposal is a complete description of a session - it
   does not specify preferences or capabilities, instead it proposes the
   media types, sources and sinks that shall be used by both endpoints
   in a session.

   Advertisements incorporate the session-related capabilities of the
   user agent(s) they describe, as modified by user policy.  A user
   agent may elect to share a customized Advertisement with a would-be
   proposer, and thus depending on the proposer, user policy may dictate
   an Advertisement offer different media types or what have.
   Advertisements may contain various proposer-specific information that
   is of use to endpoints prior to setting up a session, including
   information used in connectivity checks or keying media-layer

   Promulgating Advertisements requires some administrative pre-
   association between user agents as well as a means of requesting and
   transmitting the Advertisement itself outside context of a session.
   The existing availability of presence in many systems that establish
   multimedia sessions permits the sharing of capability and preference
   expression, as per the work in RFC5196.  Presence establishes a
   channel for communicating availability or capability information
   between presence user agents which can also share additional
   information relevant to a later stage of session establishment.  Many
   presence systems moreover have the capability to supply customized
   presence data to particular watchers, a desirable quality for the
   Advertisement/Proposal model.  An Advertisement may be shared as a
   component of an existing presence document, including a Presence
   Information Data Format (PIDF) document, or any other presence
   format.  Note that the use of presence does not necessarily entail
   the maintenance of long-term subscriptions, and might rely in some
   cases on fetch operations conducted immediately prior to session

   The Session Description Protocol provides numerous other expressions
   of capability (for example, the language of speakers in audio
   sessions) which are not considered here as requirements for
   adaptation to the Advertisement/Proposal model, though they may be

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   incorporated into applications if desired.

5.  Benefits of the Advertisement/Proposal Model

   In addition to addressing some existing problems in common SIP
   deployments, the use of the advertisement/proposal model also creates
   opportunities for new features.  Most of these new features rely on
   the availability of the Advertisement prior to the time of session
   establishment, which allows user agents or applications to consider
   or act upon this information, in non-real-time.

   The simplest application of this model is to allow the proposer the
   opportunity to browse the capabilities of an advertiser prior to
   making a session initiation attempt.  In the traditional offer/answer
   model, the user must express any preferences to his user agent prior
   to attempting to initiate a session, and only when the INVITE is
   received, and one is on the proverbial clock, does the answerer begin
   the process of determining how the offerer's capabilities and
   preferences match to its own.  While some potential methods in SIP of
   providing this "browsing" user experience prior to initiating a
   session have already been specified, as detailed above, those methods
   might turn out to be either partially redundant with the negotiation
   of the offer/answer phase or even contradictory to it - the
   Advertisement/Proposal model makes this user experience part and
   parcel of the session initiation process.

   Another example is the use of an Advertisement to predict whether or
   not connectivity will be possible behind endpoints.  Due to various
   network-layer obstacles (include NATs or firewalls), one SIP user
   agent may not be capable of forming a media session with another
   endpoint, despite their ability to rendez-vous successfully at the
   SIP layer.  In an Advertisement, a user agent might express enough
   information to allow a would-be proposer to perform ICE-style checks
   prior to initiating a session, and thus to learn whether or not a
   potential target would be reachable.  This information could be
   rendered to the proposer's user as a sort of presence information,
   for example ("red" signifying a media session cannot be formed with
   the target, "green" suggesting that it could).  This is just one
   example of a whole class of information useful during session
   establishment that could be shared during the Advertisement phase -
   encryption keys for the bodies of SIP requests are another bit of
   information that might be difficult to acquire otherwise.

   Advertisements are also useful in bunches.  For a case where a third-
   party call control system is attempting to figure out how two or more
   parties could share some sort of common session, acquiring
   Advertisements from all parties ahead of time greatly facilitates the

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   process of assembling Proposals that would be acceptable to the
   group.  Conducting this process during session establishment itself
   (by inviting each party serially to a session, for example) could
   yield far less optimal results.

   Any application that consumes Advertisement data prior to session
   establishment in order to make predictions about the session faces a
   trade-off resulting from the potential for that data to become stale.
   For an application performing a pre-session connectivity check, for
   example, changes in both the state of the presentity and the state of
   the network could render the results of a check stale at any time;
   however, the application simply can't perform connectivity checks
   constantly to mitigate this difficulty.  Moreover, since that results
   of that connectivity check are rendered to a human, nor do the checks
   even need to be performed if applications are aware that no human
   currently requires them.  Setting sensible thresholds for these sorts
   of pre-session application activities is a subject for future work,
   as are the scalability concerns arising from a watcher who follows
   Advertisements from an enormous number of presentities.

6.  The Advertisement

   This document articulates the high-level qualities of the
   Advertisement/Proposal model.  Document formats for Advertisements
   are therefore out of scope; it is possible that even the Session
   Description Protocol could be adapted to express an Advertisement or
   Proposal, provided that it can articulate the necessary elements

   Purely for exemplary purposes, an Advertisement might contain the
   following elements:
   o  Codecs and payload types supported by the originator of the
   o  Protocol support (RTP, DCCP, TCP, UDP, HTTP, etc)
   o  IP Addresses and ports where the originator may send and receive
   o  Layer coding schemes, redundant encoding and forward error
   o  Bit-rate limits, frame rates, display capabilities
   o  Security parameters of the session, including any information
      needed to key media
   o  Content meta-data and information about repositories (for file-
      transfer sessions)

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7.  The Proposal

   The Proposal is a document which describes the entire state of the
   desired session.  The construction of the Proposal may be informed by
   the Advertisement, but the interpretation of the Proposal is
   unambiguous without the Advertisement.  Again, this section merely
   sketches the high-level properties of a Proposal as an indication of
   the direction of future work.

   A Proposal may contain:
   o  A specific five tuple for each media session proposed by its
   o  A specific codec, payload type and SSRC for said media where
   o  Codec parameters (bit rates, picture sizes, etc)

8.  Backward Compatibility

   The use of the Advertisement/Proposal model integrates nicely with an
   existing deployed base of offer/anser model compliance, for the
   simple reason that one cannot send a Proposal without an
   Advertisement, and any entity that promulgates Advertisements
   supports Proposals.  For deployments where both models are in
   simultaneous use, baseline mandatory compliance with offer/answer
   would be required, though when support for Advertisement/Proposal is
   detected (via the presence of an Advertisement), it could be then
   invoked when preferred.

   At a syntactical level, any path to implementing an Advertisement/
   Proposal model requires disambiguating Proposals from offers or
   answers.  For Proposals written in SDP, perhaps a separate MIME media
   type (e.g., "application/sdp-prop") might be defined which could in
   turn be negotiated in the usual SIP matter, via OPTIONS requests, 415
   response codes, and so forth.  Any new document format for Proposals
   could of course define a new MIME type and operate in the same

9.  Security Considerations

   The Advertisement/Proposal model assumes the secure distribution of
   Advertisements from a presentity to one or more watchers with whom
   the presentity might want to share a session.  An Advertisement could
   impart secrets for keying media security, for example, which are
   intended only for a particular watcher.  The secure distribution of
   presence documents is therefore essential to this model.
   Fortunately, this problem is shared with most other sensitive

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   presence data, and thus this document need only point to the existing
   guidance for securing presence documents in RFC3863.

10.  IANA Considerations

   This document contains no considerations for the IANA.

11.  Informative References

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3265]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC3407]  Andreasen, F., "Session Description Protocol (SDP) Simple
              Capability Declaration", RFC 3407, October 2002.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 2004.

   [RFC3863]  Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
              W., and J. Peterson, "Presence Information Data Format
              (PIDF)", RFC 3863, August 2004.

   [RFC3968]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Header Field Parameter Registry for the Session
              Initiation Protocol (SIP)", BCP 98, RFC 3968,

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              December 2004.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5111]  Aboba, B. and L. Dondeti, "Experiment in Exploratory Group
              Formation within the Internet Engineering Task Force
              (IETF)", RFC 5111, January 2008.

   [RFC5196]  Lonnfors, M. and K. Kiss, "Session Initiation Protocol
              (SIP) User Agent Capability Extension to Presence
              Information Data Format (PIDF)", RFC 5196, September 2008.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

Authors' Addresses

   Jon Peterson
   NeuStar, Inc.

   Email: jon.peterson@neustar.biz

   Cullen Jennings
   Cisco Systems

   Email: fluffy@cisco.com

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